Recently, optical disks, which can record large amounts of information signals at high density, have come into use in many fields including audio, video, and computer fields. Today, when high-capacity, high-speed data transfer is becoming possible, greater packaging density is required in order to record large volumes of data such as moving-image information.
To achieve greater packaging density, there is no way but to reduce recording mark intervals and track pitches in the optical information recording medium layers. To perform reads and writes from and to an optical information recording medium with small recording mark intervals and a small track pitch, a minute laser spot is needed.
However, size of a laser spot (hereinafter referred to as a spot size) has a lower limit which depends on analysis limits, and it is not possible to reduce the spot size infinitely. Thus, an optical disk's in-plane recording density which depends on size of the recording marks or track pitch has a limit which in turn depends on the lower limit of the spot size.
To deal with this situation, there are attempts to increase information density per optical disk by increasing the number of optical information recording medium layers.
When information is read out of a multilayer optical disk using a conventional optical head, signal cross-talk may occur among the layers, resulting in large errors in a read signal. One of the causes of this is that focus position of laser light can deviate from a desired optical information recording medium layer.
A technique for eliminating disturbances in a focus error signal due to signal cross-talk is disclosed in Japanese Patent Laid-Open No. 2002-183987.
FIG. 11 is a diagram showing a configuration of such a conventional optical information recording medium and optical head. The configuration and operation will be described below with reference to FIG. 11.
Laser light emitted from a semiconductor laser 111 is converted into a parallel beam by a collimating lens 123, converted into a circular beam by a triangular prism 112, reflected by a polarizing beam splitter 131, converted into circularly polarized light by a quarter-wave plate 121, and then narrowed to a minute spot by an objective lens 141.
At focus position of the spot, a multilayer optical disk 1501 is rotating and reflected light with intensity variations is generated from optical information recording medium layers 1511, 1512, 1513, and the like which have recording marks. The reflected light returns to the objective lens 141. Then, the reflected light is converted into linearly polarized light by the quarter-wave plate 121 and transmitted through the polarizing beam splitter 131. The transmitted light is divided into two parts by a half prism 132. Reflected light from the half prism 132 is collected by a condensing lens 143 and directed onto a two-split photodetector 152 in order for electronic circuits 161 and 162 to generate a tracking error signal 172 and data signal 173.
Transmitted light from the half prism 132 is shielded by a knife edge 122, collected by a condensing lens 142, and directed onto a four-split photodetector 153. The four-split photodetector 153 includes four photodetection elements 1531, 1532, 1533, and 1534. If voltage outputs of the four photodetection elements 1531, 1532, 1533, and 1534 after current-voltage conversion are denoted by A, B, C, and D, respectively, an electronic circuit 164 performs signal processing such that the voltage outputs A, B, C, and D will satisfy the relationship E=A−B+C−D and thereby generates a focus error signal 171 which has a voltage output E.
As shown in FIG. 12, reflected light from that optical information recording medium layer 1512 of the multilayer optical disk 1501 on which the objective lens 141 is focused enters the two photodetection elements 1532 and 1533 in central part of the four-split photodetector 153 and forms a laser spot 181. The reflected light from the optical information recording medium layer 1512 which is in focus is designed to be collected and focused on a dividing line a between the photodetection elements 1532 and 1533 and enter the photodetection elements 1532 and 1533 in equal quantities so that the voltage outputs B and C generated from the two photodetection elements 1532 and 1533 after current-voltage conversion will be equal.
As shown in FIG. 13, reflected light from the optical information recording medium layer 1511 which is adjacent to the optical information recording medium layer 1512 enters the two photodetection elements 1533 and 1534 located in a lateral half of the four-split photodetector 153 and forms a laser spot 182. At this time, reflected light from the adjacent layer is designed to be focused on the left side of the dividing line a between the photodetection elements 1532 and 1533. Also, splitting position between the photodetection elements 1533 and 1534 is designed to be such that the laser spot 182 will enter the photodetection elements 1533 and 1534 in equal quantities and that the voltage outputs C and D generated from the photodetection elements 1533 and 1534 after current-voltage conversion will be equal.
This can be achieved, for example, by setting width L2 of each of the two inner photodetection elements 1532 and 1533 shorter than width L1 of each of the two outer photodetection elements 1531 and in a splitting direction among the four photodetection elements 1531, 1532, 1533, and 1534 of the four-split photodetector 153. Then, after the electronic circuit 164 performs signal processing such that the outputs of the four photodetection elements 1531, 1532, 1533, and 1534 of the four-split photodetector 153 will satisfy the relationship E=A−B+C−D to generate a focus error signal 171 which has a voltage output E, and to control a lens actuator 163 of the objective lens 141, it is possible to focus on a desired optical information recording medium layer without being affected by defocused reflected light from the adjacent optical information recording medium layers.